A common requirement in ultraviolet-visible spectroscopy is the ability to measure the specular (mirror-like) reflectance of a sample. One approach to making such a measurement is to arrange the path of a light beam between a light source and an optical detector so that there is a reflection off a mirrored surface. With the mirror in place the intensity received by the detector is noted. The mirror is then replaced with the sample and the intensity received by the detector is noted again. The change in intensity is a measure of the difference in reflectivity between the mirror and the sample. This approach has the disadvantage that it is necessary to know the reflectivity of the mirror before the reflectivity of the sample can be calculated. It is preferable that the result obtained be a direct measure of the reflectivity of the sample, not of its reflectivity relative to that of some other object. Such a direct measure is termed an absolute reflectance measurement as distinct from a relative reflectance measurement.
In known spectrometers the light source and the light detector are both fixed in position. The specular reflectance apparatus diverts the light beam onto the sample of interest and re-orients the reflected light back onto the detector. This requires a number of mirrors that both re-direct the beam and also re-focus it to allow for the change in path length between source and detector. Known approaches typically involve both fixed and re-positionable mirrors. The need for a plurality of mirrors makes such apparatus complex, and it is also undesirable because the mirrors deteriorate with handling and exposure to the atmosphere, progressively degrading the performance of the apparatus.
There is often a requirement not just to measure the absolute specular reflectance of a sample, but also to measure its absolute specular reflectance as a function of the incidence angle of the light beam. This additional requirement can add considerable complexity to conventional approaches, making implementation difficult.
The discussion of the background to the invention hereinabove is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date of the present application.
An object of the present invention is to provide relatively simple apparatus whereby the absolute specular reflectance of a sample as a function of incident angle of the light can be determined directly.